1. Field of the Invention
The present invention relates to induction motor control.
2. Description of the Related Art
One application for induction motors is in electric vehicles. In a typical electric vehicle, a plurality of batteries provide power for propulsion of the vehicle. An inverter converts the DC power provided by the batteries into AC voltages for the phases of the induction motor.
An induction motor can be controlled in "current control" mode or "voltage control" mode. In "current control", the currents provided to the phases of the motor are controlled in a feedback fashion. In "voltage control", the inverter acts as a voltage source without feedback control of the current supplied to the motor.
"Current control" and "voltage control" each have advantages and disadvantages. Current control provides fast response to transient conditions and is insensitive to motor and inverter parameter variations. However, current control typically must reserve a voltage margin to perform current regulation, thus not being capable of using the full DC bus voltage available. This represents a limitation on the average torque achievable from the motor at high speeds, if DC bus voltage is limited.
"Voltage control" is capable of using the full DC bus voltage, thus having high torque capability at high motor speeds, for a given DC bus voltage. However, voltage control can be sensitive to machine and inverter parameters and is typically an average torque control method. Thus, transient torque is somewhat difficult to control.
A system which provides the advantages of both "current control" and "voltage control" while minimizing the disadvantages of each will provide motor-control advantages over the prior art.